Mid-infrared reflectance spectra and optical constants of six iron oxide/oxyhydroxide phases

We have determined the real and imaginary indices of refraction ( n and k) for six iron oxide/oxyhydroxide phases—magnetite, maghemite, goethite, lepidocrocite, akaganéite, and ferrihydrite. A single crystal of magnetite was used to derive bulk n and k values from 100–2000 cm −1 (5–100 μm). Synthetic nanocrystalline samples of maghemite, goethite, lepidocrocite, akaganéite, and ferrihydrite were pressed into compact pellets used to determine bulk n and k values from 100–1200 cm −1 (8.33–100 μm). All values of n and k (the optical constants) were determined from specular reflectance spectra acquired at 2 cm −1 spectral sampling using classical Lorentz–Lorenz dispersion theory. In this paper, we present the optical constants of all six minerals and the oscillator parameters with which they were modeled. Use of these optical constants could aid in radiative transfer models of terrestrial dust as well as Mars, the Moon, and airless bodies in the Solar System.